Modern sedimentation and sediment dispersal pattern on the continental shelf off the Mekong River delta, South China Sea

摘要

The Mekong River is one of the major suppliers of sediments to the ocean, resulting in the formation of one of the largest river deltas. A major portion of the supplied sediments is accumulated in the subaqueous delta front, which progrades directly off the river mouths and also forms at a distance of more than 200 km westward, next to the Camau Peninsula. This study presents evidence of the existence of a Mekong-fed prodelta further offshore and provides a first quantitative assessment of the modern fluvial-derived sediment dispersal pattern to the subaqueous prodelta, the outer continental shelf and the deep region of the South China Sea. The study is based on 96 surface shelf sediment samples and five short sediment cores, which were analyzed for grain-size composition, carbonate and total organic carbon contents, sedimentary structures (X-ray radiographs), clay mineralogy and bulk geochemistry as well as ~(210)Pb and ~(137)Cs-based sediment accumulation rates. The major sediment types in water depths of 18 m to 112 m include muddy sand, sand, gravelly muddy sand, sandy mud, gravelly sand and mud. The mud is composed mostly of silt fraction, while gravel is primarily composed of shell hash. The sedimentary structures include lamination, cross-stratification, truncation surfaces and burrows of various types. The carbonate content in the sediments varies from 3% close to the Mekong River mouths to 81% further on the shelf. Total organic carbon is from 0.02% in sand to 0.94% in mud. The highest average elemental concentrations are of Si, Ca, Al, Fe, Mg and K. The spatial variability is similar for Al, K and Ti, as well as for Ca and Sr, suggesting the common presence in phyllosilicates and calcium carbonate minerals, respectively. The most common clay mineral is illite followed by smectite, kaolinite and chlorite. The spatial distribution of clay minerals suggests that they are primarily derived from the Mekong River, except in the northeastern region. The sediment accumulation rates in the mud-rich portion of the study area are in the range of 0.1 to 1.5 cm yr~(-1). According to these results, the shelf environment is divided into three zones. Westward and southward from the Camau Peninsula, the subaqueous prodelta (water depth ＜ 32 m) appears as a mud-dominated, organic-rich, high-accumulation (up to 1.5 cm yr~(-1)) zone. South of the river mouths, a wide zone is dominated by terrigenous sands, which most likely represent the sink for river-supplied bedload sediments. The third and most offshore located zone of moderate-accumulation (0.3-0.4 cm yr~(-1)) is dominated by muddy sands that are rich in biogenic carbonate. Evidence of redeposition, event deposition and changing sedimentary conditions is found in each of these zones reflecting the combined effects of tides, the changing monsoonal current and wind regimes and episodic tropical storms. The sediment budget calculation reveals that the subaqueous delta front stores approximately 50% of the fine-grained sediments supplied by the Mekong River. Roughly one-fourth of the sediments are retained in the subaerial region of the delta (including the Tonle Sap Lake), and approximately 25% accumulates on the shelf around the Camau Peninsula, primarily in the form of prodelta deposits. These results do not suggest a significant export of fine-grained sediments into the deep region of the South China Sea.